1. Field of the Invention
The present invention relates to a method of producing an external connector for a substrate. More specifically, the present invention relates to a method of producing an external connector for a substrate, which increases the number of the external connectors formed for the substrate compared to the prior art.
2. Description of the Related Art
As electronic appliances become smaller, lighter and thinner, semiconductor devices and the package thereof must also become compact. A printed circuit board (PCB), used as a substrate to connect to the semiconductor package, must be produced in an improved way so that the layout density of the PCB is increased. Currently, the opening used for forming a solder resist in the PCB has a diameter which has been decreased from about 100 microns to about 70-80 microns.
However, there still are several disadvantages in the prior art, recited as follows.
FIGS. 1-2 show a conventional external pad for a substrate in the prior art.
In FIG. 1, a plurality of external pads 102 is formed on the substrate 100. In FIG. 2, a patterned solder mask 112 is formed over the external pad 102a and the substrate 100 by exposure and development processes. The patterned solder mask 112 has a plurality of openings 112a which expose the external pad 102a. 
Here, the solder mask 112 is formed of a high-dielectric material. The solder mask 112 provides not only protection for the external pad 102a and circuits (not shown), but also electric isolation of the external pad from the circuits.
However, due to a wet process such as the development process, a tapered opening 112a tends to be formed in the solder mask 112, especially when formed in a high-dielectric material. When taking into consideration the interval of the external pads and more effective contact area at the bottom of the external pad, the number of the external pads which can be formed in the substrate is limited by the taper shape of the opening. The tapered opening makes the exposed contact area of the external pad difficult to control.
Furthermore, if non-removed solder mask resides in the opening after the development process, it is not easy to completely remove it because the external pad 102a is lower than the solder mask 112. The residual solder mask in the opening leads to poor external contact or electric connection.
In view of foregoing, there are several problems encountered in the prior art. For example, it is not easy to effectively control the external contact area of the external pad 102a. It is also difficult to increase the yield of the external pad in the substrate if the residual solder mask in the opening can not be effectively removed.
It is an object of the present invention to provide a method of producing an external connector for a substrate, which can easily control the exposed external contact area of the external pad and increase the number of the external connectors formed in the substrate.
In order achieve the above and other objects of the present invention, a method of producing an external pad for a substrate is provided. The method of the present invention includes providing a substrate and a pad layer, then globally forming a copper layer over the substrate. A patterned photoresist layer is formed over the copper layer to expose a portion of the copper layer. A conductive plug is formed on the exposed copper layer. The patterned photoresist layer is removed and then the copper layer which is not covered by the conductive plug is removed to form an external connector consisting of the conductive plug, the copper layer under the conductive plug, and the pad. A solder resist layer is formed over the substrate and the conductive plug. The solder resist layer is removed until the conductive plug is exposed, to form a solder resist there between. The solder resist is as high as the external connector.
In the present invention, a plurality of external connectors is formed and each is spaced from the other and the solder resist is formed there between. The external connector is located at the same level as the solder resist. Therefore, the external connector obtained by the present invention facilitates the subsequent processes such as wire bonding between the chip and the substrate or bump bonding between the chip and the substrate.
Furthermore, the external connector is first formed, then the solder resist is formed. It is easy to control the exposed contact area of the external connector to increase the yield of the external connector on the substrate.
Since the photoresist has a dielectric constant smaller than the solder resist, a tapered opening is formed in the solder resist more easily than in the photoresist. Accordingly, variability encountered in forming an exposed external connector can be controlled much more exactly. Meanwhile, the prior problems related to residual in the defined solder resist can be also avoided.